DE102014200080A1 - Process for producing granular polysilicon - Google Patents

Abstract

The invention relates to a process for the production of granular polysilicon in a fluidized bed reactor, comprising fluidization of silicon particles by means of a gas flow in a fluidized bed, which is heated by a heater to a temperature of 850-1100 ° C, adding a silicon-containing reaction gas and deposition of silicon on the silicon particles, removing the produced granular polysilicon from the reactor and packing the granular polysilicon, wherein from the removal of the granular polysilicon from the reactor to the packaging of the granular polysilicon, the granular polysilicon is repeatedly transferred by means of containers designed for core flow.

Description

The invention relates to a process for the preparation of granular polysilicon.

Polycrystalline silicon, often also called polysilicon for short, is produced, for example, by means of the Siemens process. Here, in a bell-shaped reactor ("Siemens reactor") thin filament rods of silicon are heated by direct current passage and a reaction gas containing a silicon-containing component and hydrogen is introduced. The filament rods are usually stuck vertically in electrodes located at the bottom of the reactor, via which the connection to the power supply takes place. Two filament rods each are coupled via a horizontal bridge (also made of silicon) and form a carrier body for silicon deposition. The bridging coupling produces the typical U-shape of the support bodies, which are also called thin rods. High-purity polysilicon deposits on the heated rods and the bridge, causing the rod diameter to increase over time (CVD / vapor deposition). After completion of the deposition, these polysilicon rods are usually further processed by mechanical processing into fragments of different size classes, classified, optionally subjected to a wet-chemical cleaning and finally packaged.

An alternative to the Siemens process are fluidized bed processes in which granular polysilicon is produced. This is done by fluidization of silicon particles by means of a gas flow in a fluidized bed, which is heated by a heater to high temperatures. By adding a silicon-containing reaction gas, a pyrolysis reaction takes place on the hot particle surface. Here, elemental silicon is deposited on the silicon particles and the individual particles grow in diameter. By the regular withdrawal of grown particles and addition of smaller silicon particles as seed particles, the process can be operated continuously with all the associated advantages. The silicon-containing educt gas described are silicon-halogen compounds (eg chlorosilanes or bromosilanes), monosilane (SiH4) and mixtures of these gases with hydrogen.

While the polysilicon obtained in the Siemens process as a cylindrical silicon rod, which must be time-consuming and costly crushed into pieces prior to further processing and possibly cleaned, has granular polysilicon bulk properties and can be used directly as a raw material such. B. are used for single crystal production for the photovoltaic and electronics industry.

In the production of granular polysilicon in a fluidized bed reactor, it is necessary in the current process to meter silicon material into the reactor at regular intervals or continuously and remove granular polysilicon from the reactor which has been finished elsewhere.

US 2011024266 A1 discloses a method of conveying granular polysilicon by means of horizontal and / or vertical movement of the conveyor, characterized in that the conveyor is completely sealed to the outside and the forward movement of the granules by a tumbling motion of the conveyor by means of the excitation of at least one mounted on the conveyor permanent magnet is generated by an electromagnetic field, wherein the electromagnetic field is applied from the outside to the encapsulated device.

The granular polysilicon must be handled at various points in the manufacturing process. First, it must be removed from the reactor. For this purpose, the method described above is suitable. Then it may need to be screened to separate it into different classes of particle sizes. For this purpose, it is transported by means of a transport container to the screening plant. Finally, the granules must be packed. Also for this purpose, it is customary to transport the granules by means of a container to the packaging plant. Alternatively, the target material from the screening plant can also be collected in a stationary container. The container is connected directly to the packaging system.

As mentioned previously, the granular polysilicon has bulk properties. Therefore, experiences from the general bulk material technology are transferable to the granular polysilicon.

One problem with the handling of bulk solids is particle segregation.

The segregation by particle size arises when a central bulk cone is formed in the middle when filling a container (or a container or a silo). When filling, the larger particles roll due to their larger mass and thus higher kinetic energy in the periphery (towards the vessel wall), while the fines accumulate primarily in the center. Such segregation across the cross-section results in product streams having different particle size distributions being discharged in succession as they exit.

If no measure is taken against particle segregation, it is obtained during production of silicon bulk materials, where the product is stored in containers, a production batch which is inhomogeneous in particle size.

For many semiconductor and photovoltaic applications, however, the most homogeneous possible particle size of the silicon raw material is required to ensure stable processes.

Bulk material is generally stored in containers or silos. If bulk material flows out of a silo, a distinction is made between mass flow and core flow.

At mass flow, the whole silo content is in motion when bulk material is being withdrawn. Mass flow is only possible if the funnel walls are sufficiently steep and / or smooth. In addition, the so-called piston flow must be achieved at the same time, in which all vertical silo sections also flow at the same speed. The essential way to accomplish this is a proper design of the hopper pitch. Only with optimal dimensioning, which is extremely difficult, the desired backmixing can be achieved.

An alternative is internal funnels, so-called Binserts. They are smaller than the actual funnel and are preceded by this. However, their use is limited and their design for use in segregation is difficult.

If the funnel wall is too shallow or too rough, the core flow will turn. At Kernfluss, initially only the bulk material in the area above the outlet opening is in motion. Bulk material in the edge area of the silo is only discharged when the silo is completely emptied. During emptying, the bulk material in the center of the silo - ie the fine material - is first removed, while at the end of the emptying, mainly coarse material is discharged. This would lead to different qualities in the individual packaging units in a downstream packaging of the bulk material.

In a mass flow silo, on the other hand, the bulk material segregated during filling flows back together in the hopper so that segregation is not felt at the outlet opening. Bulk silos usually include conical or wedge-shaped funnels.

It has also been proposed to counteract particulate separation by moving the mixing vessel. However, the great technical effort and the high wall abrasion are disadvantageous. In particular, for granular polysilicon this approach is useless because the wall abrasion leads to an undesirable contamination of high purity silicon. In addition, it can come by the movement of the bulk material to a Nachzerkleinerungseffekt, in which z. B. Dust arises.

By changing the filling process, the segregation can be minimized. It can be avoided by filling over several filling large bulk cone. Thus, the situation is defused somewhat, but not completely avoided segregation. The required complex filling system also poses a risk of contamination in the production of high-purity Si bulk solids.

Another possible solution is a discharge aid such as a controllable inner cone. The inner cone is mounted in the lower part of the container. This forms an annular gap between cone and container wall, which at the same time supplies the coarse material from the edge region and the fines from the container center to the outlet, so that there is a certain backmixing.

Another measure to influence the emptying process are so-called drainage pipes, which are equipped with gaps or holes. However, only with a sufficiently slow emptying backmixing is possible.

Mostly in the production of granular polysilicon transport containers are used, with which the material is transported from one to the next manufacturing process.

The prior art has so far offered no simple promising solutions to avoid particle segregation in the handling of granular polysilicon. Transport containers with mass flow are not practical, because because of the necessary steep Trichterneigungswinkel very large heights are needed. Since the focus is then very high up, there is a risk of tipping.

From this problem, the task of the invention resulted.

The object of the invention is achieved by a process for the preparation of granular polysilicon in a fluidized bed reactor, comprising fluidization of silicon particles by means of a gas flow in a fluidized bed, which is heated by a heater to a temperature of 850-1100 ° C, adding a silicon-containing reaction gas and Depositing silicon on the silicon particles, removing the produced granular polysilicon from the reactor, and packaging the granular polysilicon, wherein from the removal of the granular polysilicon from the reactor to the packaging of the granular polysilicon granular polysilicon is replenished several times by means of containers designed for core flow.

The invention provides that the Si bulk material is transferred several times. Preferably, it is transferred at least three times.

The containers used are designed for core flow. This is to be understood that when emptying first the granular polysilicon in the center of the container - ie the fines - is deducted, while towards the end of the discharge mainly coarse material is discharged. As a result, in contrast to transport containers with mass flow, wall abrasion and thus contamination of the granular polysilicon can be avoided.

After a certain number of Umfüllvorgängen surprisingly shows only a small Partikelentmischung on the production batch.

Further reduction of particulate matter is preferably accomplished by filling manifolds installed in the inlet of the containers. Such internals are very low contamination, preferably in silicon.

Surprisingly, it was found that, although after the first transfer a strong segregation, as described in the prior art, is present. However, if the bulk material is transferred further times, backmixing takes place so that homogenization of the particle size distribution results after a certain number of transfer steps.

The result is a granular polysilicon with a homogeneous particle size distribution over the entire production batch, wherein a deviation of the median particle sizes of any random sample from the batch is not more than 30% of the average particle size of the entire batch.

If necessary, for a further reduction of particulate separation, filling distribution cones are installed in one or more containers. These internals are very low contamination, preferably in silicon.

Preference is also the use of alternative installations such as emptying, emptying and Binsert to further minimize segregation by backmixing. The internals are made of low-contamination materials such as silicon or are lined or coated with these materials.

The number of Umfüllvorgänge in which sets as homogeneous as possible backmixing of the batch depends on the particle size distribution of the bulk material and the outlet behavior of the container. The optimum number of transfer steps is determined empirically.

The empirical determination of the transfer steps is best carried out by means of a test setup consisting of two transport containers arranged one above the other. The containers are connected by means of a container emptying or filling station and a pipeline. In addition, a sampling station is installed in the pipeline, which enables representative sampling.

Before the first transfer step, the upper container is filled with a test material having a homogeneous particle size.

During the transfer process samples are taken at regular intervals for the determination of the particle size. With the help of the results of the particle size measurements the particle segregation is determined.

The containers are exchanged before the next attempt: the full container is connected to the emptying station and the empty container to the filling station. It is recorded again the Partikelentmischung.

The experiments are repeated until a particle size as homogeneous as possible is present over the entire batch.

If required, the incorporation of a conical distributor into the containers further reduces segregation. So that the high-purity Si product is not contaminated by these internals, the invention solves this problem with a very low-contamination construction, preferably of silicon. By installing such a distributor, the formation of a large bulk cone and thus the segregation potential are reduced. Alternatively, the distributor cone can also be formed from the product itself, by forming a bulk cone on a platform below the inlet nozzle.

Preferably, the silicon particles on which silicon is deposited to produce granular polysilicon are also repeatedly transferred between the production of the silicon particles by grinding and the feeding of the silicon particles into the reactor by means of containers designed for core flow. These silicon particles, ie the seed particles in the deposition process, also have bulk material properties. For the production process of granular polysilicon, it is advantageous if the seed particles have a homogeneous particle size distribution.

Such a process for the production of polycrystalline silicon granules in a fluidized bed reactor, comprising fluidizing silicon seed particles by means of a gas flow in a fluidized bed which is heated by means of a heater, wherein by addition of a silicon-containing reaction gas by means of pyrolysis elemental silicon is deposited on the hot Keimpartikeloberflächen, whereby the polycrystalline Silicon granules formed, can be operated continuously, by removing particles grown by deposition in the diameter of the reactor and added fresh seed particles are added.

The temperature of the fluidized bed in the reaction region is preferably from 850 ° C to 1100 ° C, more preferably from 900 ° C to 1050 ° C, most preferably from 920 ° C to 970 ° C.

Hydrogen is preferably used to fluidize the seed particles.

The reaction gas is injected via one or more nozzles in the fluidized bed.

The local gas velocities at the outlet of the nozzles are preferably 0.5 to 200 m / s.

The concentration of the silicon-containing reaction gas is preferably 10 mol% to 50 mol%, particularly preferably 15 mol% to 40 mol%, based on the total amount of gas flowing through the fluidized bed.

The concentration of the silicon-containing reaction gas in the reaction gas nozzles is preferably 20 mol% to 80 mol%, particularly preferably 30 mol% to 60 mol%, based on the total amount of gas flowing through the reaction gas nozzles. The silicon-containing reaction gas is preferably trichlorosilane.

The reactor pressure moves in the range of 0 to 7 bar overpressure, preferably in the range 0.5 to 4.5 bar overpressure.

For a reactor with a diameter of z. B. 400 mm, the mass flow of trichlorosilane is preferably 200 to 400 kg / h.

The hydrogen volume flow is preferably 100 to 300 Nm ³ / h.

For larger reactors, higher amounts of TCS and H2 are preferred.

It will be understood by those skilled in the art that some process parameters are ideally selected depending on the reactor size. Also Reaktorheizleistung, seed particle dosing and bed weight are larger reactors z. Example, in a reactor with 800 mm diameter, preferably higher than the values mentioned above.

In order to clarify this clearly, the areas of the operating data normalized to the reactor cross-sectional area in which the method described in the context of this invention is valid are listed below.

The specific mass flow of trichlorosilane is preferably 1600-5500 kg / (h.m2).

The specific hydrogen volume flow is preferably 800-4000 Nm3 / (h.m2).

The specific bed weight is preferably 800-2000 kg / m 2.

The specific seed particle dosing rate is preferably 8-25 kg / (h.m2).

The specific reactor heating power is preferably 800-3000 kW / m2.

The mean diameter of the silicon particles (seed particles) is preferably at least 400 μm.

The granular polysilicon preferably has particle sizes of 150-10000 microns, wherein a mass-median value of a particle size distribution is 850-2000 microns.

The residence time of the reaction gas in the fluidized bed is preferably 0.1 to 10 s, more preferably 0.2 to 5 s.

Brief description of the figures

1 Figure 3 shows schematically the transfer operations from the removal of the granular polysilicon from the reactor to its packaging.

2 shows the particle size of the granules as a function of the filling quantity before the first transfer.

3 shows the particle size of the granules as a function of the filling quantity after the first transfer.

4 shows the particle size of the granules as a function of the filling amount after the second transfer.

5 shows the particle size of the granules as a function of the filling amount after the third transfer.

example

In the example, three refills were performed.

1 shows that the granules from the reactor 10 taken and in a buffer tank 11 is filled. Then it gets into a transport container 12 transferred to the screening plant 13 transported. Then it will turn into a buffer tank 14 transfilled and finally packed 15 ,

Transferring buffer tanks 11 in transport containers 12 corresponds to the first transfer 1 ,

The transfer of transport containers into buffer containers (via a screening plant) corresponds to the second transfer 2 ,

The transfer of buffer tank into the packaging plant corresponds to the third transfer 3 ,

In the 2 - 5 becomes the Partikelentmischung at the beginning (first buffer container after removal from the reactor, 2 ) and after each of the three refill operations.

To illustrate the segregation, the course of the particle parameters × 10, × 50 (median value) and × 90 as a function of the withdrawn Si bulk material quantity is used.

On the basis of the results obtained, the coats of coarse (large hatched) and fine (small hatched) in the 1 shown.

After the first transfer 1 there is a strong particle segregation, see 3 , The median value (× 50) starts at 980 μm. Up to a discharged amount of 320 kg, the value drops to 670 microns. Thereafter, the value increases to 1300 microns to about 880 kg of the withdrawn amounts before it then drops to about 720 microns at a further 60 kg.

After the second transfer 2 Particle segregation is already less pronounced, see 4 , The median value drops from initially just 840 to 650 μm after about 160 kg withdrawn. In the following 300 kg, the median value rises again very strongly to over 1100 microns. From 460 kg, the median decreases to approx. 790 μm. In the remaining 150 kg, the median increases again to over 920 microns.

After the third transfer 3 There is a relatively homogeneous particle size distribution over the entire batch, see 5 , The median value drops only slightly from 820 to 740 μm after 100 kg, after which there is an increase to about 1000 μm in the following 220 kg. Up to 500 kg, the value drops to 900 μm and increases to more than 1100 μm up to 700 kg. In the remaining amount of Si it decreases again and reaches a value of about 700 microns.

Due to the core flow of the containers, in which the material first flows out in the center of the container, the upper, coarse layer comes out of the container approximately in the middle of the transfer process. Since the coarse and fine particles mix again, there is apparently a partial backmixing of the production batch.

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

• US 2011024266 A1 [0006]

Claims (5)

A process for the production of granular polysilicon in a fluidized bed reactor, comprising fluidizing silicon particles by means of a gas flow in a fluidized bed which is heated to a temperature of 850-1100 ° C by a heater, adding a silicon-containing reaction gas and depositing silicon on the silicon particles, removal of the granular polysilicon produced from the reactor and packing of the granular polysilicon, wherein from the removal of the granular polysilicon from the reactor to the packaging of the granular polysilicon, the granular polysilicon is repeatedly transferred by means of containers designed for core flow. The method of claim 1, wherein at least a portion of the containers comprise filling manifolds made of silicon or coated or lined with silicon. The method of claim 1 or claim 2, wherein at least a portion of the containers comprise an emptying funnel, a discharge tube or a binsert, each made of silicon or coated or lined with silicon. The method of any one of claims 1 to 3, wherein the silicon particles on which silicon is deposited to produce granular polysilicon are produced by grinding silicon and classified by sieving, the silicon particles between generating the silicon particles and supplying the silicon particles in the reactor several times by means of containers designed for core flow. Granular polysilicon produced by a process according to one of claims 1 to 4, having a homogeneous particle size distribution over an entire production batch, characterized in that a deviation of a median particle size of any sample from the production batch is not more than 30% of the mean particle size total production batch.

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